Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Aug 20 2023 14:56:00 FOUS30 KWBC 201559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sun Aug 20 2023 Day 1 Valid 16Z Sun Aug 20 2023 - 12Z Mon Aug 21 2023 ....A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA AND SOUTHERN NEVADA... ...16Z Excessive Rainfall Discussion Update... Few changes needed along the path of Hilary in California/Nevada/Arizona...other than shaving off a few miles on the east side of the outlook area. Farther north...expanded the Slight risk area from Idaho to extend a bit more into western/southwest Montana. A quick look at the 12Z HREF probability guidance still shows a non-zero threat for 5 inch amounts (on a 40km neighborhood probability basis) in the central Idaho mountains with at least a 10 percent probability of 2 inch rainfall amounts extending into that portion of Montana. Remainder of the outlook area seemed to be well handled. Bann ....Previous Excessive Rainfall Discussion... Portions of the West... The guidance is unanimous in merging Hilary with an upper low stuck near central CA this period, which should accelerate northward across portions of the West. Normally for a tropical cyclone this would be a problem as convective lows would circle the periphery parallel to 1000-500 hPa thickness lines, which would otherwise turn it more north or north-northeast, but since Hilary should be strongly shearing while moving over cold waters and interacting with the Peninsular Ranges of the Baja California Peninsula and southern CA, it should be transitioning to a post-tropical or remnant low in the process and have diminishing convection in its vicinity; the guidance probably isn't displaying its typical model bias (this time). A large area of precipitable water values of 1.75-2.25" will approach if not exceed all-time records across portions of the Southwest, so there will be moisture to spare. In the Southwest in particular, flow at 750 hPa is expected to reach or exceed 65 kts, so heavy upslope rains on the atypical sides of the southern Sierra Nevada and Peninsula Ranges of CA are anticipated. It does appear that the flow is southerly enough for both sides of the Peninsular Ranges of southern CA to get heavy rainfall. The 19/12z NAEFS is indicating IVT values 20.2 sigmas above the mean; it should be noted that it is using a dataset that does not include the rash of tropical cyclones that impacted the Southwest in the 1970's, so this value is likely a bit too high. Even assuming a non-standard distribution and standard deviations half as large, this is extreme. There is a very real potential for 3" amounts in an hour in this environment should sufficient instability be present. Even if instability was completely eroded, 0.5" an hour totals would be possible which would be problematic for fresh burn scars; heavy rain appears inevitable. The 00z Canadian Regional shows local amounts of towards 10" while the 00z NAM CONEST advertises local amounts of 17"+, which would be exceeding rare, bordering on unique, for the region from a tropical cyclone and unique for Nevada. The 100 year ARI is forecast to be exceeded over a broad area. The 00z HREF probabilities of 5"+ (non-zero chances as far north as ID with 50%+ probabilities in NV) and 8"+ (up to 30% in portions of NV and closer to 100% near the intersection of the Peninsular and Transverse Ranges of Southern CA) are significant. It's looking increasingly probable that Hilary will be the wettest known tropical cyclone, post-tropical cyclone, or tropical cyclone remnant to impact Nevada (set in 1906), Idaho (set in 1982), and Oregon (set in 1976). Some locations within this arid region are slated to get 1-2 years worth of rain in one day. If a 7"+ maximum materialized over Mount Charleston Sunday into early Monday, which is now explicitly forecast, it would challenge Nevada's 24 hour rainfall record, set in 2004. The heavy rainfall combined with high winds expected at elevation could lead to mudslides and landslides across portions of the West, which would be exacerbated where trees uproot within saturating soils. Debris flows and rock slides are a given considering the volume of rainfall expected. The overall combination of effects could block and undermine roads, particularly sensitive areas such as sections of U.S. 50 in NV. Towns could get cut off. Flash flood emergencies are possible. Given the overall uniqueness of this event and expected impacts, the High Risk for areas of southern CA and southern NV remain justified. The main change was the joining of the two separate Slight Risk areas across the Southwest/Great Basin and southern ID (where there is a boundary to focus heavy rainfall with near record precipitable water values) and some narrowing of the risk areas in CA, NV, UT, and AZ. Florida... An easterly wave is expected to move out of FL into the Gulf of Mexico by Monday morning. Even though precipitable water values are coming down in its wake, they're still above 1.5" across western and southern FL. Inflow at 850 hPa is expected to increase to 25 kts, which should increase the effective bulk shear further and potentially lead to training convection off the Bahamas across southern FL and otherwise greater convective organization than usual. Probably due to the progression of the feature and briskness of the low-level flow, amounts weren't overly high in the guidance. However, should any convective training or mergers occur, 3" an hour totals cannot be ruled out, which would be most problematic in urban areas. Issues would likely be isolated in this scenario. Since the guidance QPF was so modest, left the risk level as Marginal. Roth Day 2 Valid 12Z Mon Aug 21 2023 - 12Z Tue Aug 22 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS OVER PORTIONS OF THE WEST... Portions of the West... Hilary's low-level circulation fades rapidly at the beginning of the period, and precipitable water values fall in suit. However, its 750 hPa circulation is on the move northward from western NV to just west of the ID borders with OR and WA. Areas of 1.5-1.75" values remain, mainly across southwest AZ and southwest ID. For southeast ID and western MT, this is remarkably anomalous and in record territory. The main focus appears to be a front/convergent boundary at 850 hPa just ahead of Hilary's 750 hPa wind circulation which is strongest during the first half of the period (Monday morning and afternoon). The guidance is insistent on 2"+ amounts, so the Slight Risk from continuity in this area looks good. Farther south across the Great Basin and Southwest, we return back towards an environment that resembles this past Friday, though with greater sensitivity due to heavy to extreme rainfall which occurs prior to Monday morning. The Slight Risk remained generally unchanged as the QPF doesn't fully tell the tale this day; some effort was made to try to include some aspect of Sunday's impacts into the Monday period and include more sensitive areas near Death Valley CA and across central NV/the US 50 corridor. It's possible that this risk area may need to be expanded or upgraded once the impacts of Sunday's potentially record rainfall become apparent. In and near the Western Great Lakes... The 12z and 00z ECMWF show the potential for heavy rainfall in this region over the top of strengthening ridging across the Plains/Midwest, the usual spot for the formation/track of an organized convective cluster. A warm front with up to 40 kts of low-level inflow causes precipitable water values to eclipse 1.5" in spots/at times. The strong veering in the low-level inflow suggests convective progression, so the main concern would be if a west-northwest to east-southeast oriented convective band formed in this area or if multiple convective rounds occurred. Hourly rain totals to 2" and local amounts to 4" would then be possible, close to what the ECMWF guidance advertises. Northwest WI, the Door Peninsula, and the northwest Mitt of MI have been wet with spots of 200-300% of average rainfall during the past week, so there should be some sensitivity there. Otherwise, issues would be restricted to urban areas. Southeast Louisiana... Now that there is an area of rotation to follow in the Florida Straits, the guidance that keeps heavy rainfall offshore the LA coast appears more believable. Coordination with LIX/the Slidell LA forecast office led to the dropping of the Marginal Risk area. Roth Day 3 Valid 12Z Tue Aug 22 2023 - 12Z Wed Aug 23 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE WEST & SOUTH TEXAS... Portions of the West... Across the interior Northwest/Northern Continental Divide, a convergence zone left in the wake of Hilary's 750 hPa circulation is expected to focus convection early on as precipitable water values fade from 1-1.5" early in the period while converging 15-25 kt winds are seen across the area. CAPE should rise to 1000 J/kg which would allow for convection with potentially high rain rates, most likely in the terrain in and near the Blue Mountains and portions of the ID Stovepipe. Continuity here was left alone as it appeared reasonable. For the Great Basin, deep layer southerly flow occurs between Hilary's 750 hPa low and the warm core ridge across MO. This keeps precipitable water values unusually high for the Great Basin, in the 1-1.25" range. Continuity had a Marginal Risk here, and that still generally looks good. There is a heavy enough of a rain signal in sensitive areas of southwest UT, which should have seen rainfall of previous days, to raise a Slight Risk. In and near Michigan... A possible convective complex from the western Great Lakes is expected to slide into the Mitt of MI this period, poleward of a warm front. A high amount of CAPE under the warm core ridge to the southwest advected in by west to west-southwest flow of close to 40 kts is expected to fuel and organize the activity. The low-level flow is veering, so there should be some convective progression. The concern would be if a training band developed or more than one round of heavy rainfall occurred. At the moment, the 00z ECMWF is most bullish with the potential, but the ingredients appear to support it. The past week has been wet, with areas of the southern Mitt of MI receiving 300%+ of their average precipitation. Introduced a Marginal Risk area here since the 00z ECMWF conceptually makes sense. South Texas... A tropical disturbance currently moving through the Florida Straits appears to develop while moving into South TX this period -- NHC currently has this system with 50% odds of becoming a tropical cyclone. Inflow at 850 hPa rises to 35+ kts, which is above the mean 850-400 hPa wind, which would lead to precipitation efficiency. Effective bulk shear would be high enough for training bands. A decent percentage of the guidance, most notably the 00z NAM, 00z UKMET, and 00z Canadian Regional, bring 5"+ amounts in a rather short time frame (within ~6 hours) across portions of South TX. Since this is a small system moving relatively quickly, if a tropical cyclone formed, areal average 3-5" with local 7" amounts would be the rule. As this is still in the formative stage, the ongoing QPF is on the low end of the 3-5" range. The Western Gulf Coast has been bone dry and the area has high FFGs. Precipitable water values of 2.5"+ with 2000+ J/kg of CAPE would potentially foster hourly rain totals to 3" in training bands or potentially near the center of the tropical low. That would overwhelm the high FFGs within two hours. Issues are expected to be mostly within urban areas. Given the above, a Slight Risk was raised for South TX, despite some lingering model spread on the latitude of the heavy rain band in South TX. Coordination with the EWX/Austin-San Antonio TX office kept the Slight Risk out of their area. Roth $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .